Cable with recyclable covering layer

ABSTRACT

A cable having at least one electrical conductor and at least one extruded covering layer based on a thermoplastic polymer material in admixture with a dielectric liquid. The thermoplastic polymer material is selected from: (a) at least one propylene homopolymer or at least one copolymer of propylene with at least one olefin comonomer; (b) a mechanical mixture of at least one propylene homopolymer or copolymer (a) and (c) at least one elastomeric copolymer or ethylene with at least one aliphatic α-olefin, and optionally a polyene.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national phase application based onPCT/EP2004/000242, filed Jan. 15, 2004, and claims the priority ofPCT/EP03/00482, filed Jan. 20, 2003, the content of both of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cable with recyclable covering layer.In particular, the invention relates to a cable for transporting ordistributing medium or high voltage electric energy, wherein an extrudedcovering layer based on a thermoplastic polymer material in admixturewith a dielectric liquid with good mechanical and electrical propertiesis present, enabling, in particular, the use of high operatingtemperatures and the transportation of high power energy.

Said cable may be used for both direct current (DC) or alternatingcurrent (AC) transmission or distribution.

2. Description of the Related Art

The requirement for products of high environmental compatibility,composed of materials which, in addition to not being harmful to theenvironment during production or utilization, may be easily recycled atthe end of their life, is now fully accepted in the field of electricaland telecommunications cables.

However the use of materials compatible with the environment isconditioned by the need to limit costs while, for the more common uses,providing a performance equal to or better than that of conventionalmaterials.

In the case of cables for transporting medium and high voltage energy,the various coverings surrounding the conductor commonly consist ofpolyolefin-based crosslinked polymer, in particular crosslinkedpolyethylene (XLPE), or elastomeric ethylene/propylene (EPR) orethylene/propylene/diene (EPDM) copolymers, also crosslinked. Thecrosslinking, effected after the step of extrusion of the polymericmaterial onto the conductor, gives the material satisfactory mechanicaland electrical properties even under high temperatures both duringcontinuous use and with current overload.

It is well known however that crosslinked materials cannot be recycled,so that manufacturing wastes and the covering material of cables whichhave reached the end of their life may be disposed of only byincineration.

Electric cables are also known having their insulation consisting of amulti-layer wrapping of a paper or paper/polypropylene laminateimpregnated with a large quantity of a dielectric liquid (commonly knownas mass impregnated cables or also oil-filled cables). By completelyfilling the spaces present in the multi-layer wrapping, the dielectricliquid prevents partial discharges arising with consequent break down ofthe electrical insulation. As dielectric liquids products are commonlyused such as mineral oils, polybutenes, alkylbenzenes and the like (see,for example, U.S. Pat. Nos. 4,543,207, 4,621,302, EP 987,718, WO98/32137).

It is however well known that mass impregnated cables have numerousdrawbacks compared with extruded insulation cables, so that their use iscurrently restricted to specific fields of application, in particular tothe construction of high and very high voltage direct currenttransmission lines, both for terrestrial and in particular forunderwater installations. In this respect, the production of massimpregnated cables is particularly complex and costly, both for the highcost of the laminates and for the difficulties encountered during thesteps of wrapping the laminate and then of impregnating it with thedielectric liquid. In particular, the dielectric liquid used must havelow viscosity under low temperatures to allow rapid and uniformimpregnation, while at the same time it must have a low tendency tomigrate during installation and operation of the cable to prevent liquidloss from the cable ends or from accidentally breaks on the cable. Inaddition, mass impregnated cables cannot be recycled and their use islimited to an operating temperature of less than 90° C.

Within non-crosslinked polymeric materials, it is known to use highdensity polyethylene (HDPE) for covering high voltage cables. HDPE hashowever the drawback of a lower temperature resistance than XLPE, bothto current overload and during operation.

Thermoplastic low density polyethylene (LDPE) insulating coverings arealso used in medium and high voltage cables: again in this case, thesecoverings are limited by a too low operating temperature (about 70° C.).

International Patent Application WO 99/13477 discloses an insulatingmaterial consisting of a thermoplastic polymer forming a continuousphase which incorporates a liquid or easily meltable dielectric forminga mobile interpenetrating phase within the solid polymer structure. Theweight ratio of thermoplastic polymer to dielectric is between 95:5 and25:75. The insulating material may be produced by mixing the twocomponents while hot either batchwise or continuously (for example, bymeans of an extruder). The resultant mixture is then granulated and usedas insulating material for producing a high voltage electric cable byextrusion onto a conductor. The material may be used either inthermoplastic or crosslinked form. As thermoplastic polymers areindicated: polyolefins, polyacetates, cellulose polymers, polyesters,polyketones, polyacrylates, polyamides and polyamines. The use ofpolymers of low crystallinity is particularly suggested. The dielectricis preferably a synthetic or mineral oil of low or high viscosity, inparticular a polyisobutene, naphthene, polyaromatic, α-olefin orsilicone oil.

International Patent Application WO 02/03398 in the name of theApplicant, discloses a cable comprising at least one electricalconductor and at least one extruded covering layer based onthermoplastic polymer material in admixture with a dielectric liquid,wherein said thermoplastic material comprises a propylene homopolymer ora copolymer of propylene with at least one olefin comonomer selectedfrom ethylene and an α-olefin other than propylene, said homopolymer orcopolymer having a melting point greater than or equal to 140° C. and amelting enthalpy of from 30 J/g to 100 J/g. Said dielectric liquidcomprises at least one alkylaryl hydrocarbon having at least twonon-condensed aromatic rings and a ratio of number of aryl carbon atomsto total number of carbon atoms greater than or equal to 0.6, preferablygreater than or equal to 0.7.

International Patent Application WO 02/27731 in the name of theApplicant, discloses a cable comprising at least one electricalconductor and at least one extruded covering layer based onthermoplastic polymer material in admixture with a dielectric liquid,wherein said thermoplastic material comprises a propylene homopolymer ora copolymer of propylene with at least one olefin comonomer selectedfrom ethylene and an α-olefin other than propylene, said homopolymer orcopolymer having a melting point greater than or equal to 140° C. and amelting enthalpy of from 30 J/g to 100 J/g. Said dielectric liquidcomprises at least one diphenyl ether, non-substituted or substitutedwith at least one linear or branched, aliphatic, aromatic or mixedaliphatic and aromatic C₁–C₃₀ hydrocarbon radical.

However, the prior art above cited presents some drawbacks.

As a matter of fact, Applicant noticed that the addition of a dielectricliquid to a polymer material should both determine a significantincrease in its electrical properties (in particular, its dielectricstrength), without impairing its thermomechanical characteristics andwithout resulting in exudation of the dielectric liquid from the polymermaterial. In particular, the resultant cable should give substantiallyconstant mechanical and electrical performances with time and hence highreliability, even at high operating temperatures (at least 90° C. andbeyond, in particular at operating temperature up to 110° C. forcontinuous use and up to 140° C. in the case of current overload). Inparticular, Applicant noticed that the presence of two phases, e.g. acontinuous phase of a thermoplastic material and an additional phaseincorporated therein of a dielectric liquid, with the consequentmicroscopically non homogeneous dispersion of said dielectric liquidonto said thermoplastic material, does not allow to obtain all the abovereported characteristics.

SUMMARY OF THE INVENTION

The Applicant has now found that it possible to overcome said drawbacksby using, as recyclable polymer base material, at least onethermoplastic propylene homopolymer or copolymer or a mechanical mixtureof said at least one thermoplastic propylene homopolymer or copolymerwith at least one elastomeric copolymer of ethylene with at least onealiphatic α-olefin, and optionally a polyene, mixed with at least onedielectric liquid as hereinafter defined. The resultant compositionpossesses suitable flexibility, excellent thermomechanicalcharacteristics and high electrical performance, such as to make itparticularly suitable for forming at least one covering layer, and inparticular an electrical insulating layer, of a medium or high voltagecable of high operating temperature, of at least 90° C. and beyond, inparticular at operating temperature up to 110° C. for continuous use andup to 140° C. in the case of current overload. The dielectric liquidsuitable for implementing the invention has high compatibility with thepolymer base material and high efficiency in the sense of improvingelectrical performance, consequently allowing the use of smallquantities (e.g. quantities lower than the saturation concentration ofthe dielectric liquid in the polymer base material) of said dielectricliquid such as not to impair the thermomechanical characteristics of theinsulating layer and to avoid the exudation of said dielectric liquidfrom the polymer base material.

High compatibility between the dielectric liquid and the polymer basematerial allows to obtain a microscopically homogeneous dispersion ofthe dielectric liquid in the polymer base material. Moreover, thedielectric liquid suitable for forming the cable covering layer of thepresent invention comprises a small quantity of polar compounds, inorder to avoid a significant increasing of the dielectric losses. It hasto be noticed also that the use of a dielectric liquid with a relativelylow melting point or low pour point (e.g. a melting point or a pourpoint not higher than 80° C.) does not give rise to manufacturingproblems both during the mixing with the polymer material and during theproduction of the cable. As a matter of fact, the low melting pointallows to an easier handling of the dielectric liquid which may beeasily melted without the need of additional and complex manufacturingsteps (e.g. a melting step of the dielectric liquid) and/or apparatuses.Moreover, Applicant noticed also that, when dielectric liquid isaromatic, high compatibility with the polymer base material may beachieved even in the presence of dielectric liquid with a low ratio ofnumber of aromatic carbon atoms to total number of carbon atoms (e.g.ratio lower than 0.6).

The Applicant has also noticed that the addition of said dielectricliquid reduces or even eliminates the optical phenomena commonly knownas “stress whitening” thanks to the fact that said dielectric liquid ismicroscopically homogeneously dispersed in the polymer material.

According to a first aspect, the present invention relates to a cablecomprising at least one electrical conductor and at least one extrudedcovering layer based on a thermoplastic polymer material in admixturewith a dielectric liquid, wherein:

-   -   said thermoplastic polymer material is selected from:        -   (a) at least one propylene homopolymer or at least one            copolymer of propylene with at least one olefin comonomer            selected from ethylene and an α-olefin other than propylene,            said homopolymer or copolymer having a melting point greater            than or equal to 130° C. and a melting enthalpy of from 20            J/g to 100 J/g;        -   (b) a mechanical mixture comprising at least one propylene            homopolymer or copolymer (a) and        -   (c) at least one elastomeric copolymer of ethylene with at            least one aliphatic α-olefin, and optionally a polyene;    -   the concentration by weight of said dielectric liquid in said        thermoplastic polymer material is lower than the saturation        concentration of said dielectric liquid in said thermoplastic        polymer material;    -   said dielectric liquid has the following characteristics:        -   an amount of polar compounds lower than or equal to 2.5% by            weight with respect to the total weight of the dielectric            liquid;        -   a melting point or a pour point lower than 80° C.;        -   a ratio of number of aromatic carbon atoms with respect to            the total number of carbon atoms lower than 0.6, when the            dielectric liquid is aromatic.

In the present description and in the subsequent claims, the term“conductor” means a conducting element as such, of elongated shape andpreferably of a metallic material, or a conducting element coated with asemiconducting layer.

The saturation concentration of the dielectric liquid in thethermoplastic polymer material may be determined by a liquid absorptionmethod on Dumbell samples: further details regarding said method will bedescribed in the examples given hereinbelow.

The amount of polar compounds of the dielectric liquid may be determinedaccording to ASTM standard D2007-02.

The melting point may be determined by known techniques such as, forexample, by Differential Scanning Calorimetry (DSC) analysis.

The pour point may be determined according to ASTM standard D97-02.

The ratio of number of aromatic carbon atoms with respect to the totalnumber of carbon atoms may be determined according to ASTM standardD3238-95(2000)e1.

According to a first embodiment, the extruded covering layer based onsaid thermoplastic polymer material in admixture with said dielectricliquid is an electrically insulating layer.

According to a further embodiment, the extruded covering layer based onsaid thermoplastic polymer material in admixture with said dielectricliquid is a semiconductive layer.

According to one preferred embodiment, the propylene homopolymer orcopolymer (a) which may be used in the present invention has a meltingpoint of from 140° C. to 170° C.

Preferably, the propylene homopolymer or copolymer (a) has a meltingenthalpy of from 30 J/g to 85 J/g.

Said melting enthalpy (ΔH_(m)) may be determined by DifferentialScanning Calorimetry (DSC) analysis.

Preferably, the propylene homopolymer or copolymer (a) has a flexuralmodulus, measured according to ASTM standard D790-00, at roomtemperature, of from 30 MPa to 1400 MPa, and more preferably from 60 MPato 1000 MPa.

Preferably, the propylene homopolymer or copolymer (a) has a melt flowindex (MFI), measured at 230° C. with a load of 21.6 N according to ASTMstandard D1238-00, of from 0.05 dg/min to 10.0 dg/min, more preferablyfrom 0.4 dg/min to 5.0 dg/min.

If a copolymer of propylene with at least one olefin comonomer (a) isused, this latter is preferably present in a quantity of less than orequal to 15 mol %, and more preferably of less than or equal to 10 mol%. The olefin comonomer is, in particular, ethylene or an α-olefin offormula CH₂═CH—R, where R is a linear or branched C₂–C₁₀ alkyl,selected, for example, from: 1-butene, 1-pentene, 4-methyl-1-pentene,1-hexene, 1-octene, 1-decene, 1-dodecene, or mixtures thereof.Propylene/ethylene copolymers are particularly preferred.

Preferably, said propylene homopolymer or copolymer (a) is selectedfrom:

-   (a₁) a propylene homopolymer or a copolymer of propylene with at    least one olefin comonomer selected from ethylene and an α-olefin    other than propylene, having a flexural modulus generally of from 30    MPa to 900 MPa, and preferably of from 50 MPa to 400 MPa;-   (a₂) a heterophase copolymer comprising a thermoplastic phase based    on propylene and an elastomeric phase based on ethylene    copolymerized with an α-olefin, preferably with propylene, in which    the elastomeric phase is preferably present in a quantity of at    least 45 wt % with respect to the total weight of the heterophase    copolymer.

Particularly preferred of said class (a₁) is a propylene homopolymer ora copolymer of propylene with at least one olefin comonomer selectedfrom ethylene and an α-olefin other than propylene, said homopolymer orcopolymer having:

-   -   a melting point of from 140° C. to 170° C.;    -   a melting enthalpy of from 30 J/g to 80 J/g;    -   a fraction soluble in boiling diethyl ether in an amount of less        than or equal to 12 wt %, preferably from 1 wt % to 10 wt %,        having a melting enthalpy of less than or equal to 4 J/g,        preferably less than or equal to 2 J/g;    -   a fraction soluble in boiling n-heptane in an amount of from 15        wt % to 60 wt %, preferably from 20 wt % to 50 wt %, having a        melting enthalpy of from 10 J/g to 40 J/g, preferably from 15        J/g to 30 J/g; and    -   a fraction insoluble in boiling n-heptane in an amount of from        40 wt % to 85 wt %, preferably from 50 wt % to 80 wt %, having a        melting enthalpy of greater than or equal to 45 J/g, preferably        from 50 J/g to 95 J/g.

Further details concerning these materials and their use in cablescovering are given in International Patent Application WO 01/37289 inthe name of the Applicant.

The heterophase copolymers of class (a₂) are obtained by sequentialcopolymerization of: i) propylene, possibly containing minor quantitiesof at least one olefin comonomer selected from ethylene and an α-olefinother than propylene; and then of: ii) a mixture of ethylene with anα-olefin, in particular propylene, and possibly with minor portions of adiene.

Particularly preferred of said class (a₂) is a heterophase copolymer inwhich the elastomeric phase consists of an elastomeric copolymer ofethylene and propylene comprising from 15 wt % to 50 wt % of ethyleneand from 50 wt % to 85 wt % of propylene with respect to the weight ofthe elastomeric phase. Further details concerning these materials andtheir use in cables covering are given in International PatentApplication WO 00/41187 in the name of the Applicant.

Products of class (a₁) are available commercially for example under thetrademark Rexflex® WL 105 of Huntsman Polymer Corporation or Borsoft® SA233 CF of Borealis.

Products of class (a₂) are available commercially for example under thetrademark Hifax® CA 10 A, Moplen® EP 310 G, or Adflex® Q 200 F ofBasell.

According to one preferred embodiment, the elastomeric copolymer ofethylene (c) has a melting enthalpy of less than 30 J/g. The quantity ofsaid elastomeric copolymer (c) is generally less than 70% by weight,preferably of from 20% by weight to 60% by weight, with respect to thetotal weight of the thermoplastic base material.

With reference to the elastomeric copolymer of ethylene (c), the term“aliphatic α-olefin” generally means an olefin of formula CH₂═CH—R, inwhich R represents a linear or branched alkyl group containing from 1 to12 carbon atoms. Preferably, the aliphatic α-olefin is selected frompropylene, 1-butene, isobutylene, 1-pentene, 4-methyl-1-pentene,1-hexene, 1-octene, 1-dodecene, or mixtures thereof. Propylene, 1-hexeneand 1-octene are particularly preferred.

With reference to the elastomeric copolymer of ethylene (c), the term“polyene” generally means a conjugated or non-conjugated diene, trieneor tetraene. When a diene comonomer is present, this comonomer generallycontains from 4 to 20 carbon atoms and is preferably selected from:linear conjugated or non-conjugated diolefins such as, for example,1,3-butadiene, 1,4-hexadiene, 1,6-octadiene, and the like; monocyclic orpolycyclic dienes such as, for example, 1,4-cyclohexadiene,5-ethylidene-2-norbornene, 5-methylene-2-norbornene, vinylnorbornene, ormixtures thereof. When a triene or tetraene comonomer is present, thiscomonomer generally contains from 9 to 30 carbon atoms and is preferablyselected from trienes or tetraenes containing a vinyl group in themolecule or a 5-norbornen-2-yl group in the molecule. Specific examplesof triene or tetraene comonomers which may be used in the presentinvention are: 6,10-dimethyl-1,5,9-undecatriene,5,9-dimethyl-1,4,8-decatriene, 6,9-dimethyl-1,5,8-decatriene,6,8,9-trimethyl-1,6,8-decatriene,6,10,14-trimethyl-1,5,9,13-pentadecatetraene, or mixtures thereof.Preferably, the polyene is a diene.

Particularly preferred elastomeric copolymers of ethylene (c) are:

-   (c_(l)) copolymers having the following monomer composition: 35 mol    %–90 mol % of ethylene; 10 mol %–65 mol % of an aliphatic α-olefin,    preferably propylene; 0 mol %–10 mol % of a polyene, preferably a    diene, more preferably, 1,4-hexadiene or 5-ethylene-2-norbornene    (EPR and EPDM rubbers belong to this class);-   (c₂) copolymers having the following monomer composition: 75 mol    %–97 mol %, preferably 90 mol %–95 mol %, of ethylene; 3 mol %–25    mol %, preferably 5 mol %–10 mol %, of an aliphatic α-olefin; 0 mol    %–5 mol %, preferably 0 mol %–2 mol %, of a polyene, preferably a    diene (for example ethylene/1-octene copolymers, such as the    products Engage® of DuPont-Dow Elastomers).

According to a preferred embodiment, the dielectric liquid has an amountof polar compounds of between 0.1 and 2.3.

According to a further preferred embodiment, the dielectric liquid has amelting point or a pour point of between −130° C. and +80° C.

According to a further preferred embodiment, the dielectric liquid has aratio of number of aromatic carbon atoms with respect to the totalnumber of carbon atoms of between 0.01 and 0.4.

According to a further preferred embodiment, the dielectric liquidpreferably has a dielectric constant, at 25° C., of less than or equalto 3.5 and preferably less than 3 (measured in accordance with IEC 247).

According to a further preferred embodiment, the dielectric liquid has apredetermined viscosity in order to prevent fast diffusion of the liquidwithin the insulating layer and hence its outward migration, as well asto enable the dielectric liquid to be easily fed and mixed into thethermoplastic polymer material. Generally, the dielectric liquid of theinvention has a viscosity, at 40° C., of between 10 cst and 800 cSt,preferably between 20 cst and 500 cSt (measured according to ASTMstandard D445-03).

According to one preferred embodiment, the dielectric liquid may beselected from: mineral oils such as, for example, naphthenic oils,aromatic oils, paraffinic oils, polyaromatic oils, said mineral oilsoptionally containing at least one heteroatom selected from oxygen,nitrogen or sulphur; liquid paraffins; vegetable oils such as, forexample, soybean oil, linseed oil, castor oil; oligomeric aromaticpolyolefins; paraffinic waxes such as, for example, polyethylene waxes,polypropylene waxes; synthetic oils such as, for example, silicone oils,alkyl benzenes (such as, for example, dodecylbenzene,di(octylbenzyl)toluene), aliphatic esters (such as, for example,tetraesters of pentaerythritol, esters of sebacic acid, phthalicesters), olefin oligomers (such as, for example, optionally hydrogenatedpolybutenes or polyisobutenes); or mixtures thereof. Paraffinic oils andnaphthenic oils are particularly preferred.

The dielectric liquid suitable for implementing the invention has goodheat resistance, considerable gas absorption capacity, in particularhydrogen absorption, and high resistance to partial discharges, so thatdielectric losses are limited even at high temperature and highelectrical gradient. The weight ratio of dielectric liquid tothermoplastic polymer material of the present invention is generallybetween 1:99 and 25:75, preferably between 2:98 and 20:80, and morepreferably between 3:97 and 10:90.

According to one preferred embodiment, the cable of the invention has atleast one extruded covering layer with electrical insulation propertiesformed from the thermoplastic polymer material in admixture with theaforedescribed dielectric liquid.

According to a further preferred embodiment, the cable of the inventionhas at least one extruded covering layer with semiconductive propertiesformed from the thermoplastic polymer material in admixture with theaforedescribed dielectric liquid. To form a semiconductive layer, aconductive filler is generally added to the polymer material. To ensuregood dispersion of the conductive filler within the thermoplasticpolymer material, the latter is preferably selected from propylenehomopolymers or copolymers comprising at least 40 wt % of amorphousphase, with respect to the total polymer weight.

According to a further aspect, the present invention relates to apolymer composition comprising a thermoplastic polymer material inadmixture with a dielectric liquid, wherein:

-   -   said thermoplastic polymer material is selected from:        -   (a) at least one propylene homopolymer or at least one            copolymer of propylene with at least one olefin comonomer            selected from ethylene and an α-olefin other than propylene,            said homopolymer or copolymer having a melting point greater            than or equal to 130° C. and a melting enthalpy of from 20            J/g to 100 J/g;        -   (b) a mechanical mixture comprising at least one propylene            homopolymer or copolymer (a) and (c) at least one            elastomeric copolymer of ethylene with at least one            aliphatic α-olefin, and optionally a polyene;    -   the concentration by weight of said dielectric liquid in said        thermoplastic polymer material is lower than the saturation        concentration of said dielectric liquid in said thermoplastic        polymer material;    -   said dielectric liquid has the following characteristics:        -   an amount of polar compounds lower than or equal to 2.5% by            weight with respect to the total weight of the dielectric            liquid;        -   a melting point or a pour point lower than 80° C.;        -   a ratio of number of aromatic carbon atoms with respect to            the total number of carbon atoms lower than 0.6, when the            dielectric liquid is aromatic.

According to a further aspect, the present invention relates to the useof a polymer composition, as described hereinabove, as the polymer basematerial for preparing a cable covering layer with electrical insulationproperties, or for preparing a cable covering layer with semiconductiveproperties.

In forming a covering layer for the cable of the invention, otherconventional components may be added to the aforedefined polymercomposition, such as antioxidants, processing aids, water treeretardants, or mixtures thereof.

Conventional antioxidants suitable for the purpose are for exampledistearyl- or dilauryl-thiopropionate and pentaerythrityl-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], or mixtures thereof.

Processing aids which may be added to the polymer composition include,for example, calcium stearate, zinc stearate, stearic acid, or mixturesthereof.

With particular reference to medium and high voltage cables, the polymermaterials as defined hereinabove may be advantageously used to obtain aninsulating layer. As stated above, these polymer base materials showindeed good mechanical characteristics both at ambient temperature andunder hot conditions, and also show improved electrical properties. Inparticular they enable high operating temperature to be reached,comparable with or even exceeding that of cables with coveringsconsisting of crosslinked polymer base materials.

If a semiconductive layer has to be formed, a conductive filler, inparticular carbon black, is generally dispersed within the polymer basematerial in a quantity such as to provide the material withsemiconductive characteristics (i.e. such as to obtain a resistivity ofless than 5 Ohm*m at ambient temperature). This quantity is generallybetween 5 wt % and 80 wt %, and preferably between 10 wt % and 50 wt %,of the total weight of the mixture.

The use of the same polymer composition for both the insulating layerand the semiconductive layers is particularly advantageous in producingcables for medium or high voltage, in that it ensures excellent adhesionbetween adjacent layers and hence a good electrical behaviour,particularly at the interface between the insulating layer and the innersemiconductive layer, where the electrical field and hence the risk ofpartial discharges are higher.

The polymer composition of the present invention may be prepared bymixing together the thermoplastic polymer material, the dielectricliquid and any other additives possibly present by using methods knownin the art. Mixing may be carried out for example by an internal mixerof the type with tangential rotors (Banbury) or with interpenetratingrotors, or, preferably, in a continuous mixer of Ko-Kneader (Buss) type,or of co- or counter-rotating double-screw type.

Alternatively, the dielectric liquid of the present invention may beadded to the thermoplastic polymer material during the extrusion step bydirect injection into the extruder cylinder as disclosed, for example,in International Patent Application WO02/47092 in the name of theApplicant.

According to the present invention, the use of the aforedefined polymercomposition in cable covering layers for medium or high voltage enablesrecyclable, flexible coverings to be obtained with excellent mechanicaland electrical properties.

Greater compatibility has also been found between the dielectric liquidand the thermoplastic polymer material of the present invention than inthe case of similar mixtures of the same polymer material with otherdielectric liquids known in the art. This greater compatibility leads,inter alia, to less exudation of the dielectric liquid. Due to theirhigh operating temperature and their low dielectric losses, the cablesof the invention can carry, for the same voltage, a power at least equalto or even greater than that transportable by a traditional cable withXLPE covering.

For the purposes of the invention the term “medium voltage” generallymeans a voltage of between 1 kV and 35 kV, whereas “high voltage” meansvoltages higher than 35 kV.

Although this description is mainly focused on the production of cablesfor transporting or distributing medium or high voltage energy, thepolymer composition of the invention may be used for covering electricaldevices in general and in particular cables of different type, forexample low voltage cables, telecommunications cables or combinedenergy/telecommunications cables, or accessories used in electricallines, such as terminals, joints or connectors.

BRIEF DESCRIPTION OF THE DRAWING

Further characteristics will be apparent from the detailed descriptiongiven hereinafter with reference to the accompanying drawing, in which:

FIG. 1 is a perspective view of an electric cable, particularly suitablefor medium or high voltage, according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, the cable (1) comprises a conductor (2), an inner layer withsemiconductive properties (3), an intermediate layer with insulatingproperties (4), an outer layer with semiconductive properties (5), ametal screen (6), and an outer sheath (7).

The conductor (2) generally consists of metal wires, preferably ofcopper or aluminium, stranded together by conventional methods, or of asolid aluminium or copper rod. At least one covering layer selected fromthe insulating layer (4) and the semiconductive layers (3) and (5)comprises the composition of the invention as heretofore defined. Aroundthe outer semiconductive layer (5) there is usually positioned a screen(6), generally of electrically conducting wires or strips woundhelically. This screen is then covered by a sheath (7) of athermoplastic material such as, for example, non-crosslinkedpolyethylene (PE).

The cable can be also provided with a protective structure (not shown inFIG. 1) the main purpose of which is to mechanically protect the cableagainst impacts or compressions. This protective structure may be, forexample, a metal reinforcement or a layer of expanded polymer asdescribed in WO 98/52197 in the name of the Applicant.

FIG. 1 shows only one possible embodiment of a cable according to theinvention. Suitable modifications known in the art can be made to thisembodiment, but without departing from the scope of the invention.

The cable covering layer or layers of thermoplastic material accordingto the present invention may be manufactured in accordance with knownmethods, for example by extrusion. The extrusion is advantageouslycarried out in a single pass, for example by the tandem method in whichindividual extruders are arranged in series, or by co-extrusion with amultiple extrusion head.

The following examples illustrate the invention, but without limitingit.

EXAMPLES 1–5

Compositions Preparation

The following components were used:

-   -   a propylene heterophase copolymer with melting point 165° C.,        melting enthalpy 30 J/g, MFI 0.8 dg/min and flexural modulus 150        MPa (Adflex® Q 200 F—commercial product of Basell);    -   a propylene heterophase copolymer with melting point 142° C.,        melting enthalpy 25 J/g, MFI 0.6 dg/min and flexural modulus of        85 MPa (Hifax® CA 10A—commercial product of Basell);    -   Sunpar® 2280 (commercial product of Sunoco): paraffinic oil with        viscosity of 475 cSt at 40° C., pour point of −15° C. and ratio        of number of aromatic carbon atoms with respect to the total        number of carbon atoms of 0.02, consisting of 69% wt paraffinic        carbon atoms, 29 wt % naphthenic carbon atoms, 2 wt % aromatic        carbon atoms and 1.5 wt % polar compounds;    -   Nyflex®820 (commercial product of Nynas): naphthenic oil with        viscosity of 110 cSt at 40° C., pour point of −27° C. and ratio        of number of aromatic carbon atoms with respect to the total        number of carbon atoms of 0.1, consisting of 10% wt aromatic        carbon atoms, 46 wt % naphthenic carbon atoms, 44 wt %        paraffinic carbon atoms and 0.2 wt % polar compounds;    -   Nytex® 840 (commercial product of Nynas): naphthenic oil with        with viscosity of 370 cSt at 40° C., pour point of −12° C. and        ratio of number of aromatic carbon atoms with respect to the        total number of carbon atoms of 0.15, consisting of 15% wt        aromatic carbon atoms, 34 wt % naphthenic carbon atoms, 51 wt %        paraffinic carbon atoms and 2.3 wt % polar compounds.

The polymer in granular form was preheated, under agitation, at 80° C.,over 15 min, in a turbomixer. Subsequently, the dielectric liquid, 6% byweight, was added to the preheated polymer. After the addition,agitation was continued for 2 hours at 80° C. until the liquid wascompletely absorbed in the polymer granules.

After this first stage, the resultant material was kneaded in alaboratory double-screw Brabender Plasticorder PL2000 at a temperatureof 180° C. to complete homogenization. The resultant material left thedouble-screw mixer in the form of granules.

Measurement of Dielectric Losses

Plates of 0.5 mm thickness were formed from the material obtained asdisclosed above. The plates were moulded at 195° C. with 15 minpreheating.

The plates obtained in this manner were subjected to dielectric lossmeasurement by measuring the tangent of the loss angle (tandelta)(according to ASTM standard D150-98) at differents temperatures (28° C.and 90° C.). The obtained results are given in Table 2.

Measurement of Flexural Modulus

The flexural modulus was determined on plates 60 mm×10 mm×1.5 mmobtained as disclosed above in accordance with ASTM standard D790-03:the obtained results are given in Table 1.

Measurement of Melting Point (T_(m)) and Melting Enthalpy (ΔH)

The melting point (Tm) and the melting enthalpy (ΔH) were determined byDifferential Scanning Calorimetry (DSC) analysis by using a MettlerToledo DSC 820 differential scanning calorimeter. The temperatureprogram below was applied to the sample to be analysed:

-   -   cooling from room temperature to −100° C.;    -   heating from −100° C. to 200° C. at a rate of 10° C./min.;    -   isotherm for 5 minutes at 200° C.;    -   cooling to −100° C. at a rate of 2° C./min.;    -   isotherm for 10 minutes at −100° C.;    -   heating to 200° C. at a rate of 10° C./min.

The obtained results are given in Table 1.

TABLE 1 Flexural Melting modulus Melting point enthalpy EXAMPLE (MPa)(T_(m)) (° C.) (ΔH) (J/g) 1 37 162 40.2 2 35 163 40.9 3 30 160 41.1 4 60139 30.7 5 60 140 32.0

TABLE 2 Gradient (G) Tandelta × 10⁻⁴ Tandelta × 10⁻⁴ EXAMPLE (kV/mm)(28° C.) (90° C.) 1 1.0 3.7 5.7 2 1.0 3.8 5.4 3 1.0 4.0 4.2 4 1.0 3.95.9 5 1.0 4.4 5.1

Example 1

94% by weight Adflex® Q 200 F+6% by weight Sunpar® 2280;

Example 2

94% by weight Adflex® Q 200 F+6% by weight Nyflex® 820;

Example 3

94% by weight Adflex® Q 200 F+6% by weight Nytex® 840;

Example 4

94% by weight Hifax® CA 10 A+6% by weight Sunpar® 2280;

Example 5

94% by weight Hifax® CA 10 A+6% by weight Nytex® 840.

EXAMPLE 6

Measurement of the Saturation Concentration

In order to determine the saturation concentration of the dielectricliquid in the thermoplastic materials, a plurality of plates weremanufactured starting from the raw materials in pellets.

Two plates (200 mm×200 mm×0.5 mm) were obtained by molding the rawmaterial (Adflex® Q 200 F) at 190° C. Five smaller Dumbell samples wereobtained from each of the above plates and weighted (W₀).

The Dumbell samples were then totally immersed at 20° C., into adielectric liquid: Sunpar® 2280 and Nyflex® 820, respectively. Thesaturation concentration was measured by determining the weight change(in percentage) of the plates after different times. The Dumbell sampleswere removed from the dielectric liquid after 3, 6, 9, 12 and 15 days,and after having cleaned their surface with a dry and clean cloth, theywere weighted (W_(i)).

The dielectric liquid absorption was determined by the followingformula:% of absorbed dielectric liquid=[(W _(i) −W ₀)/W _(i)]×100.

The saturation concentration is reached when W_(i) shows a variationlower than 1% with respect to the total weight increase which correspondto (W_(i)−W₀).

The obtained results were the following:

-   -   the saturation concentration of Sunpar® 2280 in the Adflex® Q        200 F is 25% by weight;    -   the saturation concentration of Nyflex® 820 in the Adflex® Q 200        F is 46% by weight.

EXAMPLE 7

In order to verify the absence of two phases, e.g. the absence of acontinues phase of a thermoplastic material and of an additional phaseincorporated therein of a dielectric liquid, samples of the dielectricliquid as such and of thermoplastic material additioned with thedielectric liquid were subjected to the Modulated Differential ScanningCalorimetry (MDSC) analysis using a TA Instrument DSC 2920 Modulateddifferential scanning calorimeter.

10 mg of each sample were subjected to the following temperatureprogram:

-   -   equilibrating at −145° C.;    -   modulating ±0.48° C. every 60 seconds;    -   keeping at −145° C. for 5 minutes;    -   heating to 200° C. at a rate of 5° C./min;    -   keeping at 200° C. for 2 minutes.

The obtained results are given in Table 3.

TABLE 3 EXAMPLE MDSC ANALYSIS Sunpar ® 2280 −0.59° C. Adflex ® Q 200F. + 6% absent Sunpar ® 2280 Adflex ® Q 200 F. + 34% −0.59° C. Sunpar ®2280

The results in Table 3 show that:

-   -   in the case the dielectric liquid as such, a peak at −0.59° C.        was present;    -   in the case the dielectric liquid is added to the thermoplastic        material in a quantity (6% by weight) lower than its saturation        concentration in said thermoplastic material, the peak at −0.59°        C., characteristic of the dielectric liquid as such, was not        present, showing that the dielectric liquid was microscopically        homogeneously dispersed in the thermoplastic material;    -   in the case the dielectric liquid is added to the thermoplastic        material in a quantity (25% by weight) equal to its saturation        concentration in said thermoplastic material, the peak at −0.59°        C., characteristic of the dielectric liquid as such, was        present, showing that the dielectric liquid was not        microscopically homogeneously dispersed in the thermoplastic        material.

EXAMPLES 8–9 (COMPARATIVE)

Compositions Preparation

The following components were used:

-   -   a propylene heterophase copolymer with melting point of 142° C.,        melting enthalpy 25 J/g, melting point 142° C., MFI 0.6 dg/min        and flexural modulus of 85 MPa (Hifax® CA 10A—commercial product        of Basell);    -   Nytex® 800 (commercial product of Nynas): naphthenic oil with        viscosity of 7.3 cSt at 40° C., pour point of −60° C. and ratio        of number of aromatic carbon atoms with respect to the total        number of carbon atoms of 0.07, consisting of 7 wt % aromatic        carbon atoms, 53 wt % naphthenic carbon atoms, 40 wt % of        paraffinic carbon atoms and 0.5 wt % polar compounds;    -   Indopol® L-100 (commercial product of BP Amoco): polybutene oil        with viscosity of 210 cSt at 40° C., pour point of −30° C. and        0.5 wt % polar compounds.

The polymer in granular form was preheated, under agitation, at 80° C.,over 15 min, in a turbomixer. Subsequently, the dielectric liquid, 40%by weight, was added to the preheated polymer. After the additionagitation was continued for 2 hours at 80° C. until the liquid wascompletely absorbed in the polymer granules.

After this first stage, the resultant material was kneaded in alaboratory double-screw Brabender Plasticorder PL2000 at a temperatureof 150° C. to complete homogenization. The resultant material left thedouble-screw mixer in the form of granules.

The flexural modulus, the melting point (T_(m)), the melting enthalpy(ΔH) and the dielectric losses were measured as disclosed above: theobtained results were given in Table 4 and in Table 5.

TABLE 4 Flexural modulus Melting point Melting enthalpy EXAMPLE (MPa)(T_(m)) (° C.) (ΔH) (J/g) 8 9.1 126 18.3 9 6.6 133 17.8

TABLE 5 Gradient (G) Tandelta × 10⁻⁴ Tandelta × 10⁻⁴ EXAMPLE (kV/mm)(28° C.) (90° C.) 8 1 8.9 6.1 9 1 3.3 4.6

Example 8

60% by weight Hifax® CA 10 A+40% by weight of Nytex® 800;

Example 9

60% by weight Hifax® CA 10 A+40% by weight of Indopol® L-100.

The saturation concentration of Nytex® 800 in Hifax® CA 10 A (Example 8)was determined as disclosed above and corresponds to 40% by weight.

The material of Example 8 was subjected to Modulated DifferentialScanning Calorimetry (MDSC) analysis operating as disclosed above: apeak at −93° C., characteristic of the dielectric liquid as such (namelyNytex® 800), was present, showing that the dielectric liquid was notmicroscopically homogeneously dispersed in the thermoplastic material.

EXAMPLE 10

Scanning Electron Microscopy (SEM)

Scanning Electron Microscopy (SEM) analysis was conducted as follows byutilizing the compositions of Examples 1–5 (according to the presentinvention) and the compositions of Examples 8–9 (comparative).Compression molded tensile samples were notched with a razor blade andsubsequently immersed in liquid nitrogen. Samples were then fractured ina compact tension mode. Freeze-fracture morphology of gold coatedsamples was examined with a Hitachi S-400 SEM operating at 10 KV.Digital image analysis was performed on a series of micrographs todetermine the presence of a single-phase material or of a two-phasesmaterial. At 5000× the surfaces of the samples obtained from thecompositions of Examples 1–5 (according to the present invention) werehomogeneous and devoid of cavity showing that the material is a singlephase material. On the contrary, at 5000×, the surfaces of the samplesobtained from the compositions of Example 8 and 9 (comparative), werenot homogeneous and presented a lot of cavity showing that the materialis a two phase material. Moreover, the samples obtained from Examples8–9, showed exudation of the dielectric liquid at room temperature.

EXAMPLE 11

Cable Production

The compositions of the insulating layer and of the semiconductivelayers are described in Table 6 below.

TABLE 6 Cable according to the present invention Comparison cable Innerand Inner and outer outer semicond. Insulation semicond. Insulationlayers layer layers layer (%) (%) (%) (%) by weight by weight by weightby weight Adflex ® 60.4 93.4 66.4 99.4 Q 200 F. Ensaco ® 33 — 33 — 250 GSunpar ® 6 6 — — 2280 Irganox ® 0.4 0.4 0.4 0.4 PS 802 Irganox ® 0.2 0.20.2 0.2 1010 Ensaco ® 250 G: carbon black with specific surface of 65m²/g (commercial product of MMM Carbon); Irganox ® PS 802 (antioxidant):distearyl thiodipropionate (commercial product of Ciba SpecialtyChemicals); Irganox ® 1010 (antioxidant):pentaerithrityl-tetrakis-(3-(3,5-di-t-butyl-4-hydroxy-phenyl)-propionate(commercial product of Ciba Specialty Chemicals).

The process used for manufacturing the cable was the following.

The Adflex® Q 200 F was fed directly into the extruder hopper.Subsequently, the Sunpar® 2280 previously mixed with the antioxidants,was injected at high pressure into the extruder. An extruder having adiameter of 80 mm and an L/D ratio of 25 was used. The injection wasmade during the extrusion at about 20 D from the beginning of theextrduder screw by means of three injections point on the samecross-section at 120° from each other. The dielectric liquid wasinjected at a temperature of 70° C. and a pressure of 250 bar.

The cable leaving the extrusion head was cooled to ambient temperatureby passing it through cold water.

The finished cable consisted of an aluminum conductor (cross-section 150mm²), an inner semiconductive layer of about 0.5 mm in thickness, aninsulating layer of about 4.5 mm in thickness and finally an outersemiconductive layer of about 0.5 mm in thickness.

Under similar conditions, by using the materials indicated in Table 2, acomparison cable was produced without adding the dielectric liquid.

Dielectric Strength

Three pieces (each being 20 metres in length) of the two cables producedas described above were subjected to dielectric strength measurementusing alternating current at ambient temperature. Starting from 100 kVthe gradient applied to the cables was increased by 10 kV every 10minutes until the cables broke down. The break down gradient consideredis that on the conductor.

Table 7 summarizes the results of the electrical tests: the datarepresent the average value obtained from three different measurements.

TABLE 7 Cable according to the present invention Comparison cable(kV/mm) (kV/mm) AC break down 59 29

EXAMPLE 12 (COMPARISON)

Cable Production

The compositions of the insulation layer is described in Table 8 below.

TABLE 8 COMPOSITION OF THE INSULATION LAYER (%) by weight Adflex ® Q 200F. 79.4 Sunpar ® 2280 25 Irganox ® PS 802 0.4 Irganox ® 1010 0.2

The process used for manufacturing the cable was the following.

The Adflex® Q 200 F was fed directly into the extruder hopper. Anextruder having a diameter of 80 mm and an L/D ratio of 25 was used.Subsequently, an attempt was made to inject the Sunpar® 2280 previouslymixed with the antioxidants into the extruder. The injection wasimpossible to be carried out since the dielectric liquid exit theextruder die. Consequently, the production of a finished cable wasimpossible to be carried out.

1. A cable comprising: at least one electrical conductor; and at leastone extruded covering layer based on a thermoplastic polymer material inadmixture with a dielectric liquid, wherein the thermoplastic polymermaterial is selected from: (a) at least one propylene homopolymer or atleast one copolymer of propylene with at least one olefin comonomerselected from ethylene and an α-olefin other than propylene, saidhomopolymer or copolymer having a melting point greater than or equal to130° C. and a melting enthalpy of from 20 J/g to 100 J/g; or (b) amechanical mixture comprising at least one propylene homopolymer orcopolymer (a) and (c) at least one elastomeric copolymer of ethylenewith at least one aliphatic α-olefin, and optionally a polyene; theconcentration by weight of said dielectric liquid in the thermoplasticpolymer material is lower than the saturation concentration of saiddielectric liquid in the thermoplastic polymer material; and saiddielectric liquid has the following characteristics: an amount of polarcompound lower than or equal to 2.5% by weight with respect to the totalweight of the dielectric liquid; a melting point or a pour point lowerthan 80° C.; and a ratio of number of aromatic carbon atoms with respectto the total number of carbon atoms lower than 0.6, when the dielectricliquid is aromatic.
 2. The cable according to claim 1, wherein thepropylene homopolymer or copolymer (a) has a melting point of 140° C. to170° C.
 3. The cable according to claim 1, wherein the propylenehomopolymer or copolymer (a) has a melting enthalpy of 30 J/g to 85 J/g.4. The cable according to claim 1, wherein the propylene homopolymer orcopolymer (a) has a flexural modulus, measured according to ASTMstandard D790, at room temperature, of 30 MPa to 1400 MPa.
 5. The cableaccording to claim 4, wherein the propylene homopolymer or copolymer (a)has a flexural modulus, measured according to ASTM standard D790, atroom temperature, of 60 MPa to 1000 MPa.
 6. The cable according to claim1, wherein the propylene homopolymer or copolymer (a) has a melt flowindex (MFI), measured at 230° C. with a load of 21.6 N according to ASTMstandard D1238/L, of 0.05 dg/min to 10.0 dg/min.
 7. The cable accordingto claim 6, wherein the propylene homopolymer or copolymer (a) has amelt flow index (MFI), measured at 230° C. with a load of 21.6 Naccording to ASTM standard D1238/L, of 0.4 dg/min to 5.0 dg/min.
 8. Thecable according to claim 1, wherein in the propylene copolymer (a) theolefin comonomer is present in a quantity of less than or equal to 15mol %.
 9. The cable according to claim 8, wherein in the propylenecopolymer (a) the olefin comonomer is present in a quantity of less thanor equal to 10 mol %.
 10. The cable according to claim 1, wherein in thepropylene copolymer (a) the olefin comonomer is ethylene or an α-olefinof formula CH₂═CH—R, where R is a linear or branched C₂–C₁₀ alkyl. 11.The cable according to claim 10, wherein the α-olefin is selected from:1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-decene,1-dodecene, or mixtures thereof.
 12. The cable according to claim 1,wherein the propylene homopolymer or copolymer (a) is selected from:(a₁) a propylene homopolymer or a copolymer of propylene with at leastone olefin comonomer selected from ethylene and an α-olefin other thanpropylene, having a flexural modulus generally of 30 MPa to 900 MPa; or(a₂) a heterophase copolymer comprising a thermoplastic phase based onpropylene and an elastomeric phase based on ethylene copolymerized withan α-olefin.
 13. The cable according to claim 12, wherein the propylenehomopolymer or copolymer of (a₁) has a melting point of 140° C. to 170°C.
 14. The cable according to claim 12, wherein the propylenehomopolymer or copolymer of (a₁) has a melting enthalpy of 30 J/g to 80J/g.
 15. The cable according to claim 12, wherein the propylenehomopolymer or copolymer of (a₁) has a fraction soluble in boilingdiethyl ether in an amount of less than or equal to 12 wt %, having amelting enthalpy of less than or equal to 4 J/g.
 16. The cable accordingto claim 12, wherein the propylene homopolymer or copolymer of (a₁) hasa fraction soluble in boiling n-heptane in an amount of 15 wt % to 60 wt%, having a melting enthalpy of from 10 J/g to 40 J/g.
 17. The cableaccording to claim 12, wherein the propylene homopolymer or copolymer of(a₁) has a fraction insoluble in boiling n-heptane in an amount of 40 wt% to 85 wt %, having a melting enthalpy of greater than or equal to 45J/g.
 18. The cable according to claim 12, wherein the α-olefin in theelastomeric phase of a heterophase copolymer of (a₂) is propylene. 19.The cable according to claim 12, wherein the heterophase copolymer of(a₂) is a heterophase copolymer in which the elastomeric phase consistsof an elastomeric copolymer of ethylene and propylene comprising 15 wt %to 50 wt % of ethylene and 50 wt % to 85 wt % of propylene with respectto the weight of the elastomeric phase.
 20. The cable according to claim1, wherein the elastomeric copolymer of ethylene (c) has a meltingenthalpy of less than 30 J/g.
 21. The cable according to claim 1,wherein the quantity of the elastomeric copolymer (c) is less than 70%with respect to the total weight of the thermoplastic base material. 22.The cable according to claim 1 wherein the aliphatic α-olefin in theelastomeric copolymer of ethylene (c) is an olefin of formula CH₂═CH—R,in which R represents a linear or branched alkyl group containing from 1to 12 carbon atoms.
 23. The cable according to claim 22, wherein thealiphatic α-olefin is selected from propylene, 1-butene, isobutylene,1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-dodecene, ormixtures thereof.
 24. The cable according to claim 23, wherein thealiphatic α-olefin is selected from propylene, 1-hexene, and 1-octene.25. The cable according to claim 1, wherein the polyene in theelastomeric copolymer of ethylene (c) is a conjugated or non-conjugateddiene, triene or tetraene.
 26. The cable according to claim 25, whereinthe polyene is a diene.
 27. The cable according to claim 1, wherein theelastomeric copolymer of ethylene (c) is selected from: (c₁) copolymershaving the following monomer composition: 35 mol %–90 mol % of ethylene;10 mol %–65 mol % of an aliphatic α-olefin; and 0 mol %–10 mol % of apolyene; and (c₂) copolymers having the following monomer composition:75 mol %–97 mol % of ethylene; 3 mol %–25 mol % of an aliphaticα-olefin; and 0 mol %–5 mol % of a polyene.
 28. The cable according toclaim 1, wherein the dielectric liquid comprises an amount of polarcompound between 0.1 wt % and 2.3 wt %.
 29. The cable according to claim1, wherein the dielectric liquid has a melting point or a pour pointbetween −130° C. and +80° C.
 30. The cable according to claim 1, whereinthe dielectric liquid has a ratio of number of aromatic carbon atomswith respect to the total number of carbon atoms between 0.01 and 0.4.31. The cable according to claim 1, wherein the dielectric liquid has adielectric constant, at 25° C., less than or equal to 3.5 (measured inaccordance with IEC 247).
 32. The cable according to claim 1, whereinthe dielectric liquid has a viscosity, at 40° C., between 10 cSt and 800cSt (measured according to ASTM standard D445-03).
 33. The cableaccording to claim 32, wherein the dielectric liquid has a viscosity, at40° C., between 20 cSt and 500 cSt (measured according to ASTM standardD445-03).
 34. The cable according to claim 1, wherein the dielectricliquid is selected from: mineral oils, naphthenic oils, aromatic oils,paraffinic oils, polyaromatic oils, mineral oils optionally containingat least one heteroatom selected from oxygen, nitrogen or sulphur;liquid paraffins; vegetable oils, soybean oil, linseed oil, castor oil;oligomeric aromatic polyolefins; paraffinic waxes, polyethylene waxes,polypropylene waxes; synthetic oils, silicone oils, alkyl benzenes,dodecylbenzene, di(octylbenzyl) toluene, aliphatic esters, tetraestersof pentaerythritol, esters of sebacic acid, phthalic acid esters, olefinoligomers, optionally hydrogenated polybutenes or polyisobutenes; ormixtures thereof.
 35. The cable according to claim 34, wherein thedielectric liquid is selected from paraffinic oils and naphthenic oils.36. The cable according to claim 1, wherein the weight ratio ofdielectric liquid to thermoplastic polymer material is between 1:99 and25:75.
 37. The cable according to claim 36, wherein the weight ratio ofdielectric liquid to thermoplastic polymer material is between 2:98 and20:80.
 38. The cable according to claim 37, wherein the weight ratio ofdielectric liquid to thermoplastic polymer material is between 3:97 and10:90.
 39. The cable according to claim 1, wherein the thermoplasticpolymer material is selected from propylene homopolymers or copolymerscomprising at least 40 wt % of amorphous phase, with respect to thetotal polymer weight.
 40. The cable according to claim 1, wherein theextruded covering layer based on said thermoplastic polymer material inadmixture with said dielectric liquid is an electrically insulatinglayer.
 41. The cable according to claim 1, wherein the extruded coveringlayer based on said thermoplastic polymer material in admixture withsaid dielectric liquid is a semiconductive layer.
 42. A polymercomposition comprising: a thermoplastic polymer material in admixturewith a dielectric liquid, wherein: said thermoplastic polymer materialis selected from: (a) at least one propylene homopolymer or at least onecopolymer of propylene with at least one olefin comonomer selected fromethylene and an α-olefin other than propylene, said homopolymer orcopolymer having a melting point greater than or equal to 130° C. and amelting enthalpy of from 20 J/g to 100 J/g; and (b) a mechanical mixturecomprising at least one propylene homopolymer or copolymer (a) and (c)at least one elastomeric copolymer of ethylene with at least onealiphatic α-olefin, and optionally a polyene; the concentration byweight of said dielectric liquid in the thermoplastic polymer materialis lower than the saturation concentration of said dielectric liquid insaid thermoplastic polymer material; and said dielectric liquid has thefollowing characteristics: an amount of polar compound lower than orequal to 2.5% by weight with respect to the total weight of thedielectric liquid; a melting point or a pour point lower than 80° C.;and a ratio of number of aromatic carbon atoms with respect to the totalnumber of carbon atoms lower than 0.6, when the dielectric liquid isaromatic.
 43. The polymer composition according to claim 42, wherein thepropylene homopolymer or copolymer of propylene (a) has at least one ofthe properties: a flexural modulus, measured according to ASTM standardD790, at room temperature, from 30 MPa to 1400 MPa, or a melt flow index(MFI), measured at 230° C. with a load of 21.6 N according to ASTMstandard D1238/L, from 0.05 dg/min to 10.0 dg/min.
 44. The polymercomposition according to claim 42, wherein the propylene homopolymer orcopolymer (a) is selected from: (a₁) a propylene homopolymer or acopolymer of propylene with at least one olefin comonomer selected fromethylene and an α-olefin other than propylene, having a flexural modulusgenerally of from 30 MPa to 900 MPa; and (a₂) a heterophase copolymercomprising a thermoplastic phase based on propylene and an elastomericphase based on ethylene copolymerized with an α-olefin.
 45. The polymercomposition according to claim 42, wherein the elastomeric copolymer ofethylene (c) is selected from: (c₁) copolymers having the followingmonomer composition: 35 mol %–90 mol % of ethylene; 10 mol %–65 mol % ofan aliphatic α-olefin; and 0 mol %–10 mol % of a polyene; and (c₂)copolymers having the following monomer composition: 75 mol %–97 mol %of ethylene; 3 mol %–25 mol % of an aliphatic α-olefin; and 0 mol %–5mol % of a polyene.
 46. The polymer composition according to claim 42,wherein the dielectric liquid is selected from: mineral oils, naphthenicoils, aromatic oils, paraffinic oils, polyaromatic oils, mineral oilsoptionally containing at least one heteroatom selected from oxygen,nitrogen or sulphur; liquid paraffins; vegetable oils, soybean oil,linseed oil, castor oil; oligomeric aromatic polyolefins; paraffinicwaxes, polyethylene waxes, polypropylene waxes; synthetic oils, siliconecoils, alkyl benzenes dodecylbenzene, di(octylbenzyl) toluene, alphaticesters, tetraesters of pentaerythritol, esters of sebacic acid, phthalicacid esters, olefin oligomers, optionally hydrogenated polybutenes orpolyisobutenes or mixtures thereof.
 47. A cable covering layercomprising the polymer composition according to claim 42, wherein thethermoplastic polymer material in admixture with the dielectric liquidis electrically insulating.
 48. A cable covering layer comprising thepolymer composition according to claim 42, wherein the thermoplasticpolymer material in admixture with the dielectric liquid issemiconductive.